Apparatus and method of early decoding in communication systems

ABSTRACT

A method and apparatus are disclosed for forming a frame of interleaved information bits in a communication system, where the decoding of the frame of interleaved information bits may begin before all of the bits in the frame are received. An exemplary interleaved frame is formed by receiving a frame of N information bits within the communication system; encoding the information bits at a code rate R to provide encoded bits; and arranging the encoded bits into a frame of N/R coded bits, wherein a plurality of puncturing patterns p i  are applied to the frame of N/R coded bits such that a code rate of R/a i  is produced for each of the plurality of puncturing pattern p i . The arrangement of encoded bits involves applying a puncturing pattern p j  to the encoded bits; and applying a permutation function to the punctured encoded bits to generate a fractional section of the frame of N/R coded bits. The fractional section of the frame of N/R coded bits comprises N/R * a j  bits.

RELATED APPLICATIONS

The present Application for Patent is a continuation of U.S. Ser. No.10/382,953, entitled “APPARATUS AND METHOD OF EARLY DECODING INCOMMUNICATION SYSTEMS”, filed Mar. 5, 2003, now U.S. Pat. No. 7,308,636,which is a divisional of U.S. Ser. No. 09/896,289, entitled “LOW DELAYDECODING”, filed Jun. 28, 2001, now U.S. Pat. No. 6,834,090, which is acontinuation of U.S. Ser. No. 09/305,858, entitled “LOW DELAY DECODING”,filed May 5, 1999, now U.S. Pat. No. 6,282,250, all of which areassigned to the assignee hereof and hereby expressly incorporated byreference herein.

FIELD OF THE INVENTION

This invention relates to the field of communications systems and, inparticular, to transmission of message signals in a communicationssystem.

DESCRIPTION OF RELATED ART

It is well known that it is desirable to reduce the receive and decodedelay of frames of data transmitted from a transmitter to a receiver ina communications system. For example, it is desirable to reduce framedelay when information is transmitted through a broadcast channel in acellular system. Among other things, a reduced delay will allow mobileunits to access the cellular system in a faster manner.

Some delay can be tolerated when voice telephones access a cellularsystem in order to initiate a call. In the case of cell handoff, if theamount of delay is relatively small then it is unnoticeable to the user.However, if the delay becomes too large, it can result in a call beingdropped by the system.

High data rate terminals are an example where receive and decode delayis critical. These terminals frequently go on and off line and theaccess delay can slow applications and result in congestion at the datasource. Additionally, in soft handoff even a short delay can producesignificant congestion or loss of data.

It is known in the prior art to reduce frame size in order to reducethis delay problem. However, this solution reduces the coding gain orthe amount of data that can be transmitted as well as interleaving gain.In order to partially compensate for the loss incurred using thismethod, the power of the channel can be increased or other methods suchas the R-RAKE technique can be used. Additionally increasing the powerof the channel will decrease the total capacity of the system. Moreover,the R-RAKE compensation technique requires two or more frames in a rowto be the same, and the receiver must be able to know or detect thiscondition. It can be difficult or troublesome for a system to make thisdetermination.

SUMMARY

The present invention is directed to a method and apparatus for decodinga frame of interleaved information bits in a communications system. Thepresent invention permits decoding of the frame of interleavedinformation bits before all of the bits in the frame are received at adecoding site. In the present invention, the frame of interleavedinformation bits has a frame start time and a frame end time. The framealso includes a first fractional segment that has a start time that isthe same as the frame start time and an end time that is before theframe end time. Prior to transmission of the frame of interleavedinformation bits to a decoding site, all bits in the frame are encodedat a code rate R to provide encoded bits, and the encoded bits arepositioned in the interleaved frame in accordance with an interleavingpattern that stores bits having a code rate R1 within the firstfractional segment of the interleaved frame. In a preferred embodiment,R1=R/a₁, and a₁ corresponds to the duration of the first fractionalsegment of the frame. At the decoding site, interleaved information bitsfrom the first fractional segment of the frame are initially received,and an attempt is made to decode the frame using information bits fromonly the first fractional segment of the frame. If this initial decodingattempt is unsuccessful, the system attempts to decode the frame againusing information bits from both the first fractional segment of theframe and further information bits located between the end time of thefirst fractional segment and the frame end time.

In accordance with a further aspect, the interleaving pattern describedabove also optionally stores bits having a code rate R2 at a secondfractional segment of the interleaved frame, wherein the starting timeof the interleaved frame and starting time of the second fractionalsegment coincide with each other, and the second fractional segment hasan ending time that is after an ending time of the first fractionalsegment. The interleaving pattern also optionally stores bits having acode rate R3 at a third fractional segment of the interleaved frame,wherein the starting time of the interleaved frame and starting time ofthe third fractional segment coincide with each other, and the thirdfractional segment has an ending time that is after an ending time ofthe second fractional segment. In a preferred embodiment, a₂ correspondsto a length of the second fractional segment of the frame, a₃corresponds to a length of the third fractional segment of the frame,and the interleaving pattern used is chosen such that R2=R/a₂ andR3=R/a₃. Additionally, the bits in the first fractional segment areincluded in the second fractional segment; and the bits in the first andsecond fractional segments are included in the third fractional segment.

In accordance with a still further aspect, when the interleaved frame isformed as described in the paragraph above, if the initial decodingattempt (using only the first fractional segment) is unsuccessful, thesystem attempts to decode the frame a second time using information bitsfrom only the second fractional segment of the frame. If this seconddecoding attempt fails, the system then attempts to decode the frame athird time using information bits from only the third fractional segmentof the frame. Finally, if this third decoding attempt fails, the systemattempts to decode the frame again using all information bits from theframe. In accordance with a particularly preferred aspect, the first,second and third decoding attempts described above may be performedbefore all of the frame has been received by the receiver at thedecoding site.

Many values of a₁, a₂ and a₃ may be used for implementing the presentinvention. In one embodiment described below, a₁, a₂ and a₃ correspondto the values ¼, ½, and ¾, respectively. It will be understood by thoseskilled in the art that other values for these parameters mayalternatively be used for implementing the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify corresponding elements throughout and wherein:

FIG. 1 shows a transmission and reception chain within a communicationssystem;

FIG. 2 shows the method for performing early decoding of a receivedinformation frame in accordance with the method of the presentinvention;

FIGS. 3A, 3B show an exemplary interleaving pattern used forinterleaving frames of encoded bits in accordance with the presentinvention; and

FIG. 4 shows a further method for forming an interleaved frame of codedbits for use in a communications system such as the system of FIG. 1, inaccordance with an alternative embodiment of the present invention.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown transmission chain 10 forencoding, transmitting, and decoding data in a communications channel.Information bits within transmission chain 10 are received by framingand encoding block 14, (hereinafter “block 14”), which encodes the bitsat a code rate R. As described below, the rate R is directly related tothe amount of redundancy introduced into the information stream asinformation is transformed into coded bits by block 14. After theframing and encoding is performed in block 14, coded information bitsfrom block 14 are applied to channel interleaver block 18 which outputsa frame of interleaved bits. As discussed more fully below, channelinterleaver block 18 interleaves the coded bits using an interleavingpattern that facilitates early decoding of the frame at a decoding site.More particularly, the interleaving pattern is chosen such that theinformation stored in the frame may be successfully decoded using onlybits in a first fractional portion of the interleaved frame. Theinterleaved frames output from channel interleaver block 18 arereceived, modulated and transmitted by modulation block 22. Themodulated information is transmitted by way of transmission channel 26to a receiving/decoding site.

Within a receiver of the communication system, the transmittedinformation is received and demodulated as shown in demodulation block28. The demodulated information is deinterleaved in channelde-interleaver block 32. Decoding is then performed in framing anddecoding block 36 to provide decoded information bits. As explained morefully below, framing and decoding block 36 functions by first attemptingto decode the frame using only those bits in a first fractional portionof the frame. This first decoding attempt is preferably performed beforeall the bits in a frame are received by framing and decoding block 36.If this first decoding attempt fails, framing and decoding block 36attempts again to decode the frame using additional bits from thereceived frame.

The performance of the decoding process of transmission chain 10 isrelated to the average energy per coded bit to interference ratioE_(s)/N₀ of the signal received by demodulation block 28. Theperformance of the decoding process is also related to the performanceof the code used in block 14 for coding the information bits. Onecharacteristic of the code that is important for its performance is itsrate R. The rate R is directly related to the amount of redundancyintroduced into the information stream when the information istransformed into coded bits within block 14. For example, a code of rateR=¼ produces four coded bits for each uncoded information bit. A lowercode rate R produces a more efficient code. A more efficient coderequires a lower E_(s)/N₀ for successful decoding of a frame.

In an example, the average signal-to-noise interference E_(s)/N₀ isconstant throughout a frame. A code used within framing and encodingblock 14 has a rate R. In the prior art, it is typically necessary towait until an entire frame is received before deinterleaving anddecoding it. However, it is easy to perform the deinterleaving as thesymbols are received by the receiver. Thus, there is a delay between thetime that deinterleaved bits can be available and the time when thedecoding can be performed. In order to reduce the delay in accordancewith the method of the present invention, decoding of the frame can beattempted before the reception of the frame is complete.

If channel interleaver 18 of transmission chain 10 is designed as setforth below, the decoding of the frame before the reception is completecan be performed by using only those coded bits in a first fractionalportion of the frame. For example, if the code rate R of the encodingprocess is ¼, the decoding can begin when only half of the frame isreceived. If channel interleaver 18 is appropriately chosen as describedbelow, the coded bits received in the first fractional portion of theframe appear as information bits encoded with R=¼*2=½ rate code.E_(s)/N₀ for the received bits is the same as if the entire frame hadbeen received. If the code is well designed, the performance loss istherefore only the performance loss due to converting the originalinformation bits to a higher rate code, plus half the power (3 dB)because only half the frame is received.

Referring now to FIG. 2, there is shown a graphical representation ofearly decoding method 40 of the present invention. Early decodingprocess 40 is preferably implemented on decoding block 36. In accordancewith early decoding process 40, it is possible to begin decoding a framebefore is entirely received and wait until additional bits are received.The decoding process can be attempted again once more bits have beenreceived if the previous decoding attempt failed. The repeating of thedecoding process can continue until either a successful decoding of theframe or reception of the entire frame.

In early decoding method 40, a first attempt at decoding the receivedframe is performed at time 44 using only those bits in the firstfractional portion of the frame (i.e., the first (a₁*100) % of theframe.) Assuming that the received frame was previously coded andinterleaved as described below, then at time 44 the code rate of thereceived bits is R/a₁, where a₁ represents the time between thebeginning of the frame and time 44 divided by time 52 (where time 52 isthe total duration of the frame.) If the decoding performed at time 44is not successful, another decoding of the frame can be attempted attime 48 using only those bits in the second fractional portion of theframe (i.e., the first (a₂*100) % of the frame.) Assuming again that thereceived frame was previously coded and interleaved as described belowand that the time between the beginning of the frame and time 48 is a₂,the rate of the decoded bits provided at time 48 is R/a₂, where a₂represents the time between the beginning of the frame and time 48divided by time 52. If the decoding performed at time 48 is notsuccessful, the process can then be repeated again using a thirdfractional portion of the received frame (i.e., the first (a₃*100) % ofthe frame.) Assuming that the received frame was previously coded andinterleaved as described below, then at the time of this third decodingthe code rate of the received bits is R/a₃, where a₃ represents the timebetween the beginning of the frame and the end of the third fractionalportion divided by time 52. Finally, if this third decoding attempt isunsuccessful, the frame may be decoded in a fourth attempt using all ofthe bits in the received frame. In the preferred embodiment, the firstthree decoding attempts described above may be performed before all thebits in the frame are received by framing and decoding block 36. Moreparticularly, the first decoding attempt may be performed as soon as thebits in the first fractional portion of the frame are received byframing and decoding block 36, the second decoding attempt may beperformed as soon as the bits in the second fractional portion of theframe are received by framing and decoding block 36, and the thirddecoding attempt may be performed as soon as the bits in the thirdfractional portion of the frame are received by framing and decodingblock 36.

Referring now to FIGS. 3A, 3B, there is shown an exemplary interleavingpattern used for interleaving frames of encoded bits in accordance withone embodiment of the present invention. FIGS. 3A and 3B represent anexemplary frame 61 of coded bits output from encoding block 14. In theexample shown, the frame is 16 bits in length, the bits are numbered0-15, and the code used for coding the bits in frame 61 has rate R. Eachframe of coded bits from block 14 is then applied to channel interleaver18, which interleaves the bits in accordance with an interleavingpattern that facilitates early decoding of each received frame at adecoding site as described above. An exemplary interleaving pattern thatfacilitates such early decoding is shown in connection with FIG. 3B,which illustrates an exemplary interleaved frame 62 output by channelinterleaver block 18. In the example shown, the first fractional portion(a₁*100%) of the interleaved frame corresponds to the first ¼ of theinterleaved frame 62 and includes bits coded at the code rate R1, whereR1=R/a₁; the second fractional portion (a₂*100%) of the interleavedframe corresponds to the first ½ of the interleaved frame 62 andincludes bits coded at the code rate R2, where R2=R/a₂; and the thirdfractional portion (a₃*100%) of the interleaved frame corresponds to thefirst ¾ of the interleaved frame 62 and includes bits coded at the coderate R3, where R3=R/a₃. This interleaving pattern may be used inconnection with an early decoding process that permits decoding attemptswhen ¼, ½, and ¾, respectively, of the frame is received by framing anddecoding block 36. It will be understood by those skilled in the artthat other values of a₁, a₂ and a₃ may be used for implementing thepresent invention, and that each frame may be interleaved using only oneor two or more than three of the fractional portions described above. Itwill also be understood by those skilled in the art that otherinterleaving patterns besides that shown in FIG. 3B may be used forimplementing the early decoding process of the present invention, andthat the use of such of interleaving patterns is within the scope of thepresent invention.

Referring now to FIG. 4, there is shown a further method for forming aninterleaved frame of coded bits, in accordance with an alternativeembodiment of the present invention. In the method shown in FIG. 4, eachframe of N information bits is applied to an encoder 66 that codes theinput bits using a code rate R. For each frame of N information bitsapplied to encoder 66, the encoder outputs a frame having N/R codedbits. Puncturing patterns p_(i) are then applied to each frame of codedinformation bits at block 74. A code of rate R/a_(i) is thereby producedfor each puncturing pattern p_(i). The values of a_(i) correspond to allof the possible fractions of the information frame at which detection isattempted within early decoding method 40. For example, a₁=¼, a₂=½, a₃=¾can be used. This corresponds to attempting to decode when ¼, ½, and ¾,respectively, of the frame is received. This selection can be performedusing brute force techniques. For example, the values ¼, ½, and ¾ can betried and the results determined. Other values can then be tried and theresults determined. Each puncturing pattern p_(i) is chosen so that thepunctured coded bits contain the coded bits obtained using puncturingpattern p_(j) if j<i. Thus, the bits produced by puncturing with thefirst pattern must be included in bits produced by subsequent (larger)patterns.

Channel interleaver 18 can then be selected in view of the puncturingpatterns p_(i) as follows. Information bits 64 are applied to encoder66. Information bits 64 have a length N. Encoder 66 has a rate R.Encoded bit stream 70 having a length N/R is thus provided at the outputof encoder 66. Puncturing device 74 receives encoded bit stream 70 andapplies a puncturing pattern p_(i) to it. The output of puncturingdevice 74 is applied to permutation function 78. Permutation function 78can be any permutation function that provides N/R*a_(i) output bits 82as shown within interleaver selection method 60.

Output bits 82 are inserted by channel interleaver 18 within firstfraction 90 of frame duration 86. First fraction 90 has a length a₁*100%the length of frame duration 86. If repeated attempts are made to decodethe frame, N/R*a₂ output bits 82 and N/R*a₃ output bits 82 frompermutation function 78 must be inserted into the appropriate fractionsof frame duration 86 by channel interleaver 18. The inserted output bits82 have lengths of a₂*100 percent and a₃*100, respectively, of thelength of frame duration 86 and have a starting time that coincides withframe duration 86.

The previous description of the preferred embodiments is provided toenable a person skilled in the art to make and use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein can be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosed.

1. A method for forming an interleaved frame in a communications system,comprising: receiving a frame of N information bits within thecommunication system; encoding the information bits at a code rate R toprovide encoded bits; arranging the encoded bits into a frame of N/Rcoded bits, wherein a plurality of puncturing patterns p_(i) are appliedto the frame of N/R coded bits such that a code rate of R/a_(i) isproduced for each of the plurality of puncturing patterns p_(i), whereina_(i) represents a fractional portion of the frame of N/R coded bitsdecodable prior to receipt of all of the coded bits of the frame of N/Rcoded bits; and transmitting the frame of N/R coded bits.
 2. The methodof claim 1, wherein a_(i) comprises at least one of ¼, ½ and ¾.
 3. Themethod of claim 1, wherein the step of arranging the encoded bitsfurther comprises: applying a first puncturing pattern p_(j) to theencoded bits; applying a permutation function to the punctured encodedbits to generate a fractional section of the frame of N/R coded bits. 4.The method of claim 3, wherein the fractional section of the frame ofN/R coded bits comprises a first number, N/R*a_(j), of bits.
 5. Themethod of claim 4, wherein the bits produced by puncturing with thefirst puncturing pattern p_(j) is included in the bits produced bypuncturing with a second puncturing pattern p_(j+1).
 6. An apparatus forforming an interleaved frame in a communications system comprising: anencoder that receives a frame of N information bits within thecommunication system and encodes the information bits at a code rate Rto provide encoded bits; an interleaver that arranges the encoded bitsinto a frame of N/R coded bits, wherein each of a plurality ofpuncturing patterns p_(i) are applied to the frame of N/R coded bitssuch that a code rate of R/a_(i) is produced for each of the pluralityof puncturing patterns p_(i), wherein a_(i) represents a fractionalportion of the frame of N/R coded bits decodable prior to receipt of allof the coded bits of the frame of N/R coded bits; and a transmitter thattransmits the frame of N/R coded bits.
 7. The apparatus of claim 6,wherein a_(i) comprises at least one of ¼, ½ and ¾.
 8. The apparatus ofclaim 6, wherein the step of arranging the encoded bits furthercomprises: applying a first puncturing pattern p_(j) to the encodedbits; applying a permutation function to the punctured encoded bits togenerate a fractional section of the frame of N/R coded bits.
 9. Theapparatus of claim 8, wherein the fractional section of the frame of N/Rcoded bits comprises a first number, N/R*a_(j), of bits.
 10. Theapparatus of claim 9, wherein the bits produced by puncturing with thefirst puncturing pattern p_(j) is included in the bits produced bypuncturing with a second puncturing pattern p_(j+1).
 11. An apparatusfor forming an interleaved frame in a communications system comprising:means for receiving a frame of N information bits within thecommunication system; means for encoding the information bits at a coderate R to provide encoded bits; means for arranging the encoded bitsinto a frame of N/R coded bits, wherein a plurality of puncturingpatterns P_(i) are applied to the frame of N/R coded bits such that acode rate of R/a_(i) is produced for each of the plurality of puncturingpatterns p_(i), wherein a_(i) represents a fractional portion of theframe of N/R coded bits decodable prior to receipt of all of the codedbits of the frame of N/R coded bits; and means for transmitting theframe of N/R coded bits.
 12. The apparatus of claim 11, wherein a_(i)comprises at least one of ¼, ½ and ¾.
 13. The apparatus of claim 11,wherein the step of arranging the encoded bits further comprises:applying a first puncturing pattern p_(j) to the encoded bits; applyinga permutation function to the punctured encoded bits to generate afractional section of the frame of N/R coded bits.
 14. The apparatus ofclaim 13, wherein the fractional section of the frame of N/R coded bitscomprises a first number, N/R*a_(j), of bits.
 15. The apparatus of claim14, wherein the bits produced by puncturing with the first puncturingpattern p_(j) is included in the bits produced by puncturing with asecond puncturing pattern p_(j+1).